In the present time, transformers are primarily divided into two main categories. One of these two categories is an asymmetrical winding transformer as shown in Taiwanese patent No. M289515. The other category includes a symmetrical planar transformer as shown in Taiwanese patent publication No. 200839800.
Taiwanese patent No. M289515 discloses a transformer. The transformer includes a first winding rack and a second winding rack. There is a central hole disposed in the center of the first winding rack for containing a central column of an E-shaped magnetic core. A coil is wound around the periphery of the first winding rack. A casing opening having a larger inner diameter is disposed in one end of the first winding rack. There is also a central hole disposed in the center of the first winding rack, for containing a central column of another E-shaped magnetic core. Another coil is wound around the periphery of the second winding rack. One end of the second winding rack is inserted into the casing opening of the first winding rack, so as to form a retractable or telescoping structure. A plurality of conductive pins of the first and second winding rack is disposed on the other side of the retractable structure for connecting with the coils. By sliding the end of the second winding rack in the casing opening of the first winding rack to change the clearance between the two coils, the value of the resonant inductance or leakage inductance can be adjusted.
Taiwanese patent publication No. 200839800 discloses a symmetrical planar transformer. The transformer includes a first circuit board, a second circuit board, and a coil connecting element. The first circuit board has a first coil and a second coil. The second circuit board is disposed corresponding to the first circuit board and has a third coil and a fourth coil. The coil connecting element has a first pin and a second pin. The first pin is connected with the first coil and the second pin is connected with the second coil. The coil connecting element controls the connecting or separation between the first coil and the second coil.
In Taiwanese patent No. M289515, the coil is wound by using traditional winding method. Furthermore, the transformer must have larger volume to permit the second winding frame to slide so that it cannot be manufactured in a compact size. In Taiwanese patent publication No. 200839800, the coils are disposed in the circuit board, so that the transformer cannot be manufactured in a more compact size. Furthermore, a composite magnetic material can be disposed between the first circuit board and the second circuit board. By controlling the thickness of the composite magnetic material, the resonant inductance can be adjusted. However, because of the increasing demand for a more compact electronic device and the increasing cost reduction pressures as experienced by companies, the symmetrical planar transformer is even more difficult to meet the demands of the market.